Mission impossibel: We have to burn less than 25% of known fossil fuel reserves to limit temperature rise to 2°C

Two possible futures: One in which no climate policies are implemented (red), and one with strong action to mitigate emissions (blue). Shown are fossil CO2 emissions (top panel) and corresponding global warming (bottom panel). The shown mitigation pathway limits fossil and land-use related CO2 emissions to 1000 billion tonnes CO2 over the first half of the 21st century with near-zero net emissions thereafter. Greenhouse gas emissions of this pathway in year 2050 are ~70% below 1990 levels. Without climate policies, global warming will cross 2 °C by the middle of the century. Strong mitigation actions according to the blue route would limit the risk of exceeding 2°C to 25%. Credit: M. Meinshausen et al. (2009)

Study Concludes That to Limit Global Warming to 2 °C, Less Than 25% of Proven Fossil Fuel Reserves Can be Burnt Between Now and 2050

Less than a quarter of the proven fossil fuel reserves can be burnt and emitted between now and 2050, if global warming is to be limited to two degrees Celsius (2 °C), according to a new study published in the journal Nature today. This issue of Nature—themed “The Coming Climate Crunch”—features a number of related papers and commentary on greenhouse gas emissions and the difficulty of cutting back, as well as an editorial calling on commitment from “the highest levels” to make the needed changes.

The study, led by Malte Meinshausen at the Potsdam Institute for Climate Impact Research (PIK), calculated how much greenhouse gas emissions can be pumped into the atmosphere between now and 2050 to have a reasonable chance of keeping warming lower than 2 °C (above pre-industrial levels)—a goal supported by more than 100 countries to prevent dangerous climate change.

The researchers, involving scientists from Germany, the United Kingdom and Switzerland, concluded that the limit is 1,000 billion tonnes of carbon dioxide between the years 2000 and 2050. The world has already emitted one third of that in just nine years.

The three-year study concluded that greenhouse gas emissions must be cut by more than 50% by 2050 relative to 1990 levels, if the risk of exceeding 2 °C is to be limited to 25%.

If we continue burning fossil fuels as we do, we will have exhausted the carbon budget in merely 20 years, and global warming will go well beyond two degrees. Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming. We shouldn’t forget that a 2°C global mean warming would take us far beyond the natural temperature variations that life on Earth has experienced since we humans have been around.

—Malte Meinshausen

The study also compared the volume of CO2 emissions that could result from the burning of known economically recoverable fossil fuel reserves—oil, gas and coal—and found that these reserves are four times larger than the emission budget between now and 2050.

To keep warming below 2 °C, we cannot burn and emit the CO2 from more than a quarter of the economically recoverable fossil fuels up to 2050, and in the end only a small fraction of all known fossil fuel reserves.

—Bill Hare, co-author of the study

The study used a single, efficient computer model which incorporated the effects of all greenhouse gases, aerosols and air pollutants, and the range of possible responses of the carbon cycle and earth’s climate system. This was combined with about a thousand emission pathways.

The study explicitly takes into account the uncertainties related to modeling climate change. Throughout the study, probability statements were used to summarize the current level of knowledge based on observational data. It also used a huge number of different simulation results from the latest assessment report of the Intergovernmental Panel on Climate Change. In taking this comprehensive approach the researchers went a step further than previous work.

The new results have direct relevance to the international negotiations now underway.

With every year of delay, we consume a larger part of our emissions budget, losing room to manoeuvre and increasing the probabilities of dangerous consequences.

—Reto Knutti, co-author from the ETH Zurich

A companion study, also published in Nature today by Myles Allen and colleagues, shows the necessity to limit the total amount of carbon that humankind ever emits.

In principle, it is the sum of all CO2 emissions that matters. In practice, substantial reductions in global emissions have to begin soon, before 2020. If we wait any longer, the required phase-out of carbon emissions will involve tremendous economic costs and technological challenges—miles beyond what can be considered politically feasible today. The longer we wait, the more likely our path will lead us into dangerous territory.

—Malte Meinshausen

The authors of both papers collaborated on a Commentary article focusing on their long-term policy implications, published today in Nature Reports Climate Change.